1. Field of the Invention
This invention relates generally to machines and procedures for printing text or graphics on a printing medium such as paper, transparency stock, or other glossy media; and more particularly to a scanning thermal-inkjet machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs print-mode techniques to optimize image quality and operating time, and also to minimize distortion of the image and of the printing medium.
2. Related Art
In a multiple-pen printer, it is important to as economically and simply as possible maximize both the output quality of a printed page and the speed at which that output can be obtained.
In a printer mechanism, the output quality of a printed page is a function of printhead resolution. The finer is the resolution, the better the print quality.
Also, in a swath printer (e.g., one employing a scanning carriage with a pen capable of printing multiple pixel rows concurrently) the speed at which the output can be obtained is a function of the height of the swath which is covered by the printhead.
In multipen printers, until making of the invention covered by the Harris et al. document mentioned above, each pen had the same resolution and usually the same swath width. This meant that increasing resolution of any part of the system would require scaling up all the supporting structure, mechanics and electronics to support the resolution of the entire set of pens.
Thus heretofore in designing printing machines it has been necessary to confront difficult choices between high speed, high resolution and other print-quality characteristics, and economy. The aforementioned Harris, Azmoon and Nobel document, however, teaches that many such choices can be advantageously sidestepped.
Harris et al. provide within a single printer plural writing devices of different color, speed, swath height and resolution. The printer is programmed to operate the writing devices in a way that takes advantage of the swath height, speed and resolution characteristics of each writing device to obtain an enhanced mix of speed, resolution and economy.
For example, if one pen creates a relatively tall, high-resolution swath and is loaded with black ink, that pen can be used for relatively rapid throughput of black text or graphics alone. The printer is thereby capable of serving in the stead of a relatively fast, high-resolution black-text or black-only-graphics printer.
The same pen can be used for relatively rapid throughput of the black component of color images. Other pens, loaded with color inks--or another single pen capable of discharging different color inks--in the same printer can be used to form the chromatic components of color images. Thus addition of a relatively small amount of hardware enables the printer to do color work as well as fast, high-resolution black printing.
Providing just one pen of desired higher resolution and greater swath height--and providing in the same set other pens of lower resolution and lesser swath height--creates a plural-resolution, plural-swath-width system. Such a hybrid system is far less expensive than the full, major hardware scale-up, mentioned above, that would be needed for an all-high-resolution, all-high-swath-height printer.
Thus in the interest of economy the color pen or pens can be limited in capability to creation of a relatively shallow, lower-resolution swath. Most interestingly, such economy is not severely deleterious to printed results, inasmuch as the perceptual capability,of the human eye is relatively insensitive to detail in chromatic features. Moreover, chromatic colors in a large fraction of practical cases (particularly in business graphics) are used only to provide color fill in relatively large, uniform fields.
Thus the invention of Harris, Azmoon and Nobel incorporate plural-resolution capabilities directly into the printer printheads, expanding the capabilities of the printer to achieve high-quality printing as well as greater throughput. Their invention decreases research and development costs as well as decreasing the time for bringing higher-resolution printers to market.
Harris et al. provide a color printer having one basic printhead resolution for color printing and a different basic printhead resolution for monochrome printing such as black printing. In a preferred form, a higher basic printhead resolution is provided for monochrome printing (particularly black) and a lower for color (particularly cyan, magenta and yellow. They integrate these black and color printing components into the same printing mechanism, providing composite printing off higher-resolution black and lower- resolution color concurrently.
Harris et al. furthermore provide increased throughput for the higher-resolution monochrome component of the color printer. In their preferred form, a taller-swath monochrome printhead such as a high-resolution black printhead which produces approximately dots of suitable size for spacing at 23.6 dots per mm (600 dots per inch, or "dpi") is mounted on the same carriage as shallower-swath color printheads such as lower-resolution cyan, magenta and yellow printheads which produce approximately 11.8 dots/mm-sized (300-dpi-sized) printout dots.
The taller-swath black printhead has overlapping printing alignment with all of the shallower-swath color printheads. The taller-swath black printhead has a three-hundred-nozzle swath with a nozzle pitch of about 0.042 mm (1/600 inch), to create a swath of approximately 12.7 mm (one-half inch), and the shallower-swath color printheads each have a hundred-nozzle swath with a nozzle pitch of about 0.085 mm (1/300 inch) to create a swath of approximately 8.5 mm (one-third inch).
The availability of plural writing devices of different character in a single printer, however, does not--in and of itself--cure every problem of swath-based printing technology. Some such problems that remain are outlined below.
Furthermore, under certain circumstances the use of plural writing devices of different character in a single printer can introduce undesired subtle patterning effects, of a sort not found in printers using only matched writing devices. In particular, when printing devices of different resolution are used together, dot-placement errors can differently affect printing at the different resolutions, degrading print quality at one or another resolution.
The present invention relates to certain of the remaining problems, and also certain subtle degradations that are inherent in the use of different plural writing devices together.
(a) Ink-flux effects--To achieve vivid colors in inkjet printing with aqueous inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the water base--by evaporation (and, for some printing media, absorption)--and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise: so-called "bleed" of one color into another (particularly noticeable at color boundaries that should be sharp), "blocking" or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and "cockle" or puckering of the printing medium.
These problems are well known in the art. Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects.
(b) Staggered pens--Colors can be separated during printing by use of staggered, i.e. vertically offset, pens. Such an approach is relatively undesirable because it requires a bigger carriage and a bigger printer, and also introduces additional complexity in mutual alignment of the several pens.
(c) Prior heat-application techniques--Among these techniques is heating the inked medium to accelerate evaporation of the water base or carrier. Heating, however, has limitations of its own; and in turn creates other difficulties due to heat-induced deformation of the printing medium.
Glossy stock warps severely in response to heat, and transparencies too can tolerate somewhat less heating than ordinary paper. Accordingly, heating has provided only limited improvement of drying characteristics for these plastic media.
As to paper, the application of heat and ink causes dimensional changes that affect the quality of the image or graphic. Specifically, it has been found preferable to precondition the paper by application of heat before contact of the ink; preheating, however, causes loss of moisture content and resultant shrinking of the paper fibers. Shrinkage is commonly nonuniform and creates gross distortions of the medium and naturally its image. Through closer control of the printing medium and the image segments near the ends of the pages, such problems have been mitigated but not entirely eliminated.
(d) Prior print-mode techniques--Another useful technique is laying down in each pass of the pen only a fraction of the total ink required in each section of the image--so that any areas left white in each pass are filled in by one or more later passes. This tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time.
Print modes have been designed to minimize the conspicuousness of image distortions arising in various ways. The aforementioned patent document of Cleveland presents an extensive discussion of some print modes and the problems they attack; a brief discussion appears shortly in another subsection hereunder.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a "print model". Heretofore print modes have been substantially the same for all printheads (for example, all pens) used at any one time in each printer--and accordingly have been substantially the same for all colors printed concurrently in a given printer.
(e) "Concurrent" printing--As used in this document, the term "concurrently" is intended to encompass ongoing continuing operations of a generally unitary printing machine, such as for example (1) printing one color during one direction of scanning of a pen carriage, and forthwith printing another color in another direction of scanning; or (2) printing one group, e.g. some specified number of rows, of pixels in one set of passes across a print medium, and then printing another group of pixels that are interspersed among the first group, in another set of passes. Thus the word "concurrently" encompasses, but does not require, printing "simultaneously".
As used in this document, however, the word "concurrently" excludes such operations as printing one element (one color, or one group of pixels) for an entire page, or for an entire image, and then printing another element (another color or another group of pixels) for the same entire page or image. This sort of printing is excluded by the term "concurrently" whether the successive elements are printed on one generally unitary printing machine or on more than one such machine.
(f) Resolution--Furthermore printheads used together have had common size and provided common resolution. Accordingly partial-inking patterns have been substantially the same for the resolutions of all pens.
In this regard it is important to have a clear understanding of what is meant by the resolution of a pen, for purposes of the present document. High quality printers are typically characterized by numbers indicating their is resolution in dots per millimeter (dots/mm) or dots per inch (dpi). This resolution is usually described by a pair of numbers, in the context of a two-dimensional coordinate system--where one number indicates the resolution along the x-axis (as used herein, x-axis means the axis of carriage scanning for a swath printer), and another number indicates the resolution in the v-axis (as used herein, v-axis means the axis of printing-medium advance for a swath printer). Thus, a resolution of 11.8/11.8 dots/mm (300/300 dpi) generally indicates a carriage-scan axis resolution of 11.8 dots per millimeter (300 dots per inch) and a printing-medium-advance axis resolution of 11.8 dots per millimeter (300 dots per inch).
The term "resolution" means ability to resolve or separate--usually to separate visually--two image elements or details. In one sense, resolution of a print-off, head is primarily determined by the actual printout dot size as it appears in a printout, since perceptual separation is difficult for two large dots even if their centers are geometrically displaced, by some distance smaller than their radii. So in one ideal theoretical world, an 11.8 dot/mm (300 dpi) printhead is presumed to produce a printout dot size which is approximately 0.085 mm (1/300 inch) in diameter.
In another and more fundamental sense, however, dot size can be subordinated to center-to-center spacing, since resolution finer than center-to-center spacing is a technical impossibility regardless of dot size. Therefore, various common language usages have developed which define resolution in other closely related terms. For example, the resolution of a printhead is often identified by its nozzle pitch (i.e., the distance between adjacent nozzles on a printhead), and a print mode resolution is often identified by its pixel addressability (i.e., the distance between adjacent pixels in a printout).
There are several print mode techniques for enhancing the print-quality characteristics of a printhead. For example, an 11.8 dot/mm (1/300 inch) nozzle-pitch printhead can be used to create a 23.6 dot/mm (600 pixel/inch) printout along the print-medium-advance axis by changing the incremental advance distance of the medium at the end of a swath and then employing a multipass print mode.
Such a system which provides printout pitch that is finer than the hardware pitch is sometimes called an "addressable" fine-pitch or high-resolution system. As another example, an 11.8 dot/mm (1/300 inch) nozzle-pitch printhead could be used to create a 23.6 dot/mm (600 pixel/inch) printout along the carriage-scan axis by suitably choosing the firing frequency of the printhead or the carriage scan speed, or both.
Implementing these different print modes, however, is rather complicated and requires sophisticated programming techniques, precisely engineered mechanical parts, and many circuit components. Moreover, the print quality of a lower-resolution machine which has a 23.6 dot/mm (600 dpi) "addressable" print mode is not as good as the print quality of a true 23.6 dot/mm-resolution (600-dpi-resolution) machine in which both smallest dot size and addressability are each equal to 23.6 dots/mm (600 dpi).
Some print modes such as square or rectangular checkerboard-like patterns tend to create objectionable moire effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise, for example, in dithering subsystems sometimes used to help control the paper advance or the pen speed.
(g) Known technology of print modes: general introduction--One particularly simple way to divide up a desired amount of ink into more than one pen pass is the checkerboard pattern mentioned above: every other pixel location is printed on one pass, and then the blanks are filled in on the next pass.
To avoid horizontal "banding" problems (and sometimes minimize the moire patterns) discussed above, a print mode may be constructed so that the paper advances between each initial-swath scan of the pen and the corresponding fill-swath scan or scans. In fact this can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
Once again this technique tends to distribute rather than accumulate print-mechanism error that is impossible or expensive to reduce. The result is to minimize the conspicuousness of--or, in simpler terms, to hide--the error at minimal cost.
(h) Print masks vs. inking locations--Masking relates to addressability of pixel positions in each operation of a printing device (for example, each pass of a transversely scanning pen). Actual inking (or actual addressing) of particular pixels is thus a very different matter from masking.
Actual inking depends upon not only (1) addressability but also (2) the desired-image data which a particular user supplies to the printer--in effect, the image detail which the user wishes to see in the vicinity of each pixel--and (3) various special procedures, known as "rendition", employed to translate desired image details into a pattern of ink dots which the printer can produce.
Thus desired-image data and rendition procedures typically prevent actual inking of most colors (or, in many systems, almost all colors) in each pixel position. This is true even for a pixel which is addressable for all, or almost all, colors if only print masking alone is considered.
Conversely, in any given operation of a given printing device (e.g., pen pass) even if image data and rendition call for printing a particular color at a particular pixel, that color may not be printed at that pixel. Depending on the masking scheme in use, that color may have been printed there already in a previous operation, or may be reserved for printing there in a later operation, of that same (or another) printing device.
(i) Print masks vs. unit quantity of colorant--In some devices heretofore, different quantities (for example, a different number of drops) of ink are provided for different colors. This is particularly true for different types of printing media.
Thus in printing on transparency stock (plastic sheets used for making, as an example, masters that can be projected on a screen by an overhead projector) it is necessary to provide much more colorant than when printing on paper. Similarly glossy stock (plastic-coated paper popular as cover sheets for reports or looseleaf books) requires more colorant than ordinary paper requires, to produce a like impression of color vividness.
Variable inking in such situations, whether for different colors or different media, or different crosscombinations of the two, is produced in rendition. Here too, this is not a matter of masking--which relates only to assigning particular color-to-pixel applications to particular printing-device operations. In all such situations, the masking heretofore is the same for all printing devices and all colors within any given printer.
The same is true whether the unit quantity for each color is one dot of colorant, or plural dots, or even fractional dots (as described for example in the aforementioned patent document of Askeland et al.). It is also true whether printing is binary (go or no-go, for each color) or plural-level (for instance any number of color-quantity units from zero to 2.sup.n, for each color, where n is the number of data bits in the color-specification system). In all these variants, masking is common to all colors and all printing devices used concurrently within any one printer.
(j) Print-mode masks: space- and sweep-rotated, and autorotating--The pattern used in printing each nozzle section is known as the "print-mode mask". The term "print model" is more general, usually encompassing a description of a mask, the number of passes required to reach full density and the number of drops per pixel defining "full density".
Operating parameters can be selected in such a way that, in effect, rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes. Figuratively speaking this can be regarded as "automatic" rotation or simply "autorotation".
The Cleveland patent document mentioned earlier discusses these techniques at greater length.
(k) Conclusion--As pointed out above, availability of plural writing devices of different character in a single printer has left some problems remaining to be solved in swath-based printing technology. As also noted above, use of plural writing devices of different character in a single printer can itself introduce subtle undesired effects not found in printers using matched writing devices exclusively.
Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.